1. Field of the Art
The present invention relates to a semiconductor device with bump electrodes, and more particularly to a semiconductor device with bump electrodes formed on electrode pads.
2. Prior Art
A conventional bump structure of a semiconductor device and a method of forming a bump will be described with reference to FIGS. 5A to 5D.
Referring to FIG. 5A, a semiconductor substrate 21 with impurities diffused and an SiO.sub.2 film formed on the surface thereof (the impurity diffused region and the film are not shown) is formed thereon with an Al electrode pad 22. An insulating passivation film (Si.sub.3 N.sub.4 film) 23 with an opening formed at the position above the Al electrode pad 22 is deposited on the semiconductor substrate 21 and on a part of the electrode pad 22. A metal film 24 as barrier metal layer is deposited on the Si.sub.3 N.sub.4 film 23 and electrode pad 22 by means of the vapor deposition method or the sputtering method.
Next, as shown in FIG. 5B, a photopolymerized high polymer dry film 25 is pressure-laminated on the metal film 24 with a hot laminator under the conditions of 105.degree. C. roll temperature, 0.2 m/min roll feed speed, and 20 Kg/cm.sup.2 roll pressure. The dry film 25 is subjected to patterning to form an opening with a size corresponding to a desired bump side length, at the position only above the electrode pad 22. Thereafter, the patterned dry film 25 is baked at 100.degree. C. for 10 minutes.
Next, by using the patterned dry film 25 as a plating mask, and the metal film 24 as one (a cathode) of the electrodes (cathode) in electrolytic plating, Au is selectively plated only within the opening of the dry film 25.
After removing the dry film 25, a cubical Au deposit 26 is obtained on the metal film 24 above the electrode pad 22, as shown in FIG. 5C. Thereafter, as shown in FIG. 5D, by using the Au deposit 26 as a mask, the metal film 24 except that above the electrode pad is etched out. Thus, a semiconductor device with the cubical Au bump 26 of a conventional structure formed on the metal film 24 above the Al electrode pad 22 is obtained.
Instead of the dry film, a liquid resist film may be used as a plating mask. However, in this case, it is difficult to deposit a plating mask thicker than the height of an Au bump which is 20 to 30 .mu.m. As a result, an Au bump necessarily has a mushroom shape. In contrast, by using a dry film it becomes possible to obtain a plating mask of 20 to 40 .mu.m thicker than the height of an Au bump, so that an Au bump can have a cubical shape. Thus, it is advantageous that a relatively larger number of Au bumps per unit can be formed on the semiconductor substrate.
With the conventional method using a dry film, however, a portion 24a of the metal film 24 under the Au bump 26 is generally etched inward by about 5 .mu.m in the etching process of the metal film 24, using the Au bump as a mask. Consequently, the peel strength of the Au bump 26 is as low as 30 to 40 g/pad (with the area of the electrode pad being 110.times.110 .mu.m.sup.2, and the area of the opening being 80.times.80 .mu.m.sup.2). In order to obtain a sufficient peel strength, it becomes necessary to enlarge the dimension of the Au bump, which prevents full use of the advantages of the method using a dry film.
Further, since the metal film 24 is etched inward by about 5 .mu.m, the peripheral portion of the Al electrode pad 22 is covered with only the Si.sub.3 N.sub.4 film 23a. As a result, the corrosion resistance of the Al electrode pad is lowered so that the reliability becomes degraded.